The nature of the cellular targets sensitive to lithium and sodium toxicity represents an important gap in our knowledge on the physiology of ion homeostasis in eukaryoUc cells. The characterisation of these targets is essential for the understanding of clinical problems such as the effects of lithium on the therapy for dipolar disorder [Schou (1997) Arch, Gen. Psychiatry 54, 9] or high sodium levels associated with hypertension [Lifton (1996) Science 272, 676). Another problem, completely different but also related to ionic homeostasis, is the progressive salinisation of cultivated lands subjected to intensive irrigation, which has turned crop plant breeding for salt tolerance into an urgent need for the development of a sustainable agriculture in arid regions [Serrano (1996) Int. Rev. Cytol. 165; 1; Yeo (1998) J. Exp. Bot. 49, 915; Holmberg & Bülow (1998) Trends Plant Sci. 3, 61].
Apart from ion transport [Haro et al. (1991) FEBS Lett. 291, 189; Gaxiola et al. (1999) Proc. natl. Acad. Sci. USA 96, 1480; Apse et al. (1999) Science 285, 1256] and osmolyte synthesis [Tarczynski et al (1993) Science 259, 508; Kishor et al. (1995) Plant Physiol. 108, 1387: Alia et al. (1998) Plant J. 16, 155], the manipulation of cellular systems most sensitive to high ion concentrations and to water stress offers alternative routes to improve salt tolerance of crop plants [Serrano (1996) Int. Rev. Cytol. 165, 1; Tezara et al. (1999) Nature 401, 914].
Genetic and biochemical analyses have allowed to identify the product of the yeast gene HAL2 as an important physiological target of salt toxicity [Gläser et al. (1993) EMBO J. 12, 3105; Dichtl et al. (1997) EMBO J. 16, 7184]. HAL2 encodes a 3′,5′-biphosphate nucleotidase, which is very sensitive to inhibition by lithium and sodium [Murguía et al. (1995) Science 267, 232]. Salt inhibition of Hal2p results in the intracellular accumulation of 3′-phosphoadenosine 5′-phosphate (pAp) [Murguía et al. (1996) J. Biol. Chem. 271, 29029], a toxic compound which in turn inhibits the reactions of reduction and transfer of sulphate groups, as well as some exoribonucleases [Dichtl et al. (1997) EMBO J. 16, 7184; Gil-Mascarell et al. (1999) Plant J. 17, 373]. There are genes homologous to HAL2 in plants [Gil-Mascarell et al. (1999) Plant J. 17, 373] and in mammals [López-Coronado et al. (1999) J. Biol. Chem. 274, 16043] although in the latter case the encoded enzyme is inhibited by lithium but not by sodium.
The salt tolerance conferred by overexpression of Hal2p, the wild-type protein as well as mutated versions resistant to lithium and sodium, is relatively modest [Albert et al. (2000) J. Mol. Biol. 295, 927]. This suggests the existence of additional targets of salt toxicity, which become limiting once the HAL2 bottleneck is overcome, but the nature of these important salt-sensitive processes is not yet known.
Two patent applications relating to osmotic stress but describing protective mechanisms different to the general mechanism described in the present invention are the following: ES2110918A (1998-02-16) relating to the production of plants tolerant to osmotic stress through the manipulation of carbohydrate metabolism and ES2134155A1 (1998-10-07) relating to a method to confer tolerance to osmotic, water and salt stress in glycophylic plants, through the use of genes encoding proteins with peroxidase activity.